1. Field of the Invention
The present invention relates to a welding laser focal point position detecting apparatus and in particular, to a laser focal point position detecting apparatus for use when welding metal plates.
2. Description of the Related Art
Conventionally, a laser is applied to a welding position on two metal plates placed one on the other, so that the two metal plates at that position are melted and attached to each other. Especially, this laser welding is used for welding a sheet steel. The welding quality depends on a deviation amount from an optimal position of the laser focal point. For obtaining a preferable welding, it is necessary to control the focal point to be at an optimal position.
In a conventional laser welding apparatus, a distance between a laser converging means or parabolic reflector and a plate to be welded is detected, and the position of the converting means is controlled, so that the distance is constant. Thus, the position of the laser focal point position is corrected. This configuration is disclosed in Japanese Patent Application A4-127983.
In this example, however, the laser focal point position is corrected, assuming that the distance from the converging means to the laser focal point is constant. (That is, the laser focal point position is not directly detected.) Accordingly, if the distance between the converging means and the laser focal point is changed due to deterioration with age of an optical system and laser, it becomes impossible to correct the focal point to be at its optimal position.
Moreover, U.S. Pat. No. 5,607,605 discloses a technique to photograph a plasma with a CCD camera and calculates a plasma image-occupied ratio in the picture photographed. However, in this example, the relationship between the plasma image-occupied ratio and the focal distance is not monotonous increase or decrease, and it is impossible to decide whether it is nearer or farther from the optimal focal point position.
It is therefore an object of the present invention to provide a welding laser focal point position detecting apparatus capable of directly detecting a welding laser focal point independently of the position of the converging means for converging laser.
Moreover, the present invention detects the focal point at real time, so that the focal point is always at its optimal position.
Furthermore, the present invention enables to detect the focal point at a very small interval smaller than one second so that a focal point position change can be detected immediately.
In order to attain the aforementioned, the inventor of the present invention performed an experiment as follows. An aluminum plate was placed on another aluminum plate and a laser beam (carbon dioxide laser) was applied to a welding position. Thus, melting state was checked. This check was repeated while changing the converging means of reflector or lens in a direction of the laser beam and the position of the converging means which gave the maximum melting was made to be a reference position of the laser focal point. The converging means has parabolic reflector or converging lens.
Here, the distance between the converging means and the laser focal point is constant if no change with age is considered for the optical system. Next, a photo element was fixed at a predetermined distance from the melting position to receive the plasma light emission from the welding position. Outputs of the photo element was sampled periodically for a predetermined time interval and the light intensity data items thus obtained were used to calculate a variance value.
The converging means was positioned so that the laser focus was at the reference position. Furthermore, the position of the converging means was successively shifted by 1 mm so that the laser focal point was displaced by 1 mm each time. Variance values were calculated for the range of minus 2 to plus 3 mm with respect to the reference position. This resulted in the graph shown in FIG. 4. The horizontal axis represents a laser focal point position assuming 0 for the reference position. The vertical axis represents variance values.
From this experiment, it has been found that in a predetermined range (minus 2 to plus 2 mm), there is one-to-one relationship between the variance value and the focal point. Utilizing this relationship, the inventor found a method to determine a focal point position from a variance.
The apparatus according to the present invention comprises: a photo element for receiving light from a position of welding performed by a laser beam source and converting the intensity of the received light into an electric signal; a sampler for sampling periodically the electric signal produced from the photo element, periodically with a predetermined interval; and a calculator for calculator for calculating a data distribution (deviation from a reference value) or variance of discrete light intensity data items when a predetermined number of light intensity data items have been stored.
The apparatus may further comprise: a focal point table which specifies a laser focal point position corresponding to a variance; and a focal point position calculation means for referencing the variance of focal point table obtained by the calculator, so as to convert the variance of the light intensity data into a welding laser focal point position for output.
Moreover, the inventor used an average value of the light intensity data in addition to the variance. That is, a characteristic amount of the variance divided by a square of the average value. As a result, it was found that the characteristic amount and the focal point position was in the relationship shown in FIG. 6.
This FIG. 6 shows a wider range (minus 2 to plus 3 mm) of one-to-one correspondence between the vertical axis as the characteristic amount and the horizontal axis of the focal point position in comparison to the relationship between the variance value and the focal point position
Accordingly, if a focal point position is derived from the aforementioned characteristic amount, it is possible to detect a focal point position in a wider range than when using the correspondence between the variance and the focal point position.
The welding laser focal point position detecting apparatus according to the present invention comprises: a photo element for receiving light from a position of welding performed by a laser beam source and converting the intensity of the received light into an electric signal; a sampler for sampling periodically the electric signal produced from the photo element, periodically with a predetermined interval; and variance value calculation means for calculating a variance value of discrete light intensity data obtained by the sampler.
Moreover, the apparatus according to the present invention may comprise: an average value calculation means for calculating an average value of the discrete light intensity data obtained by the sampler; a characteristic amount calculation means for calculating a characteristic amount of a variance divided by a square of the average value; and a focal point table for specifying a laser focal point according to the characteristic amount.
Furthermore, the apparatus may comprise a focal point position calculation means for referencing the focal point table so as to convert the characteristic amount obtained by the characteristic amount calculation means, into a welding laser focal point for output.